Doc: update doc of heartbeat metric #359

Job	Run time
lint	12m 6s
openraft-feature-test (nightly)	54s
examples (stable, memstore)	37s
openraft-feature-test (nightly, serde)	1m 3s
examples (stable, raft-kv-memstore)	2m 34s
openraft-feature-test (nightly, single-term-leader)	52s
openraft-test (stable, 0)	3m 27s
examples (stable, raft-kv-memstore-network-v2)	53s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	50s
openraft-test (nightly, 30)	3m 29s
cluster-benchmark (cluster_benchmark)	1m 5s
examples (stable, raft-kv-memstore-opendal-snapshot-data)	1m 1s
openraft-test (nightly, 0, single-term-leader)	3m 17s
examples (stable, raft-kv-memstore-singlethreaded)	50s
openraft-test-bench (nightly)	1m 1s
examples (stable, raft-kv-rocksdb)	8m 26s
rt-monoio	45s
examples (nightly, memstore)	54s
stores (stores/memstore)	55s
examples (nightly, raft-kv-memstore)	2m 16s
Build (nightly, bench,serde,bt,singlethreaded)	52s
stores (stores/rocksstore)	6m 20s
examples (nightly, raft-kv-memstore-network-v2)	1m 8s
tests-feature-test (nightly)	3m 3s
stores (stores/sledstore)	1m 1s
examples (nightly, raft-kv-memstore-opendal-snapshot-data)	1m 23s
tests-feature-test (nightly, single-term-leader)	3m 3s
examples (nightly, raft-kv-memstore-singlethreaded)	1m 20s
tests-feature-test (nightly, loosen-follower-log-revert)	3m 1s
examples (nightly, raft-kv-rocksdb)	12m 29s
	1h 20m 55s

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